1. Prepare the marinade by combining the soy sauce, olive oil, chilli flakes and a pinch of salt in a small bowl.
2. Place the salmon on a plate and pour over all the marinade, ensuring both side of the fillets are covered. Cover and set aside to marinate for 10 minutes.
3. On an oven tray, place two sheets of foil to make a cross, the foil must be large enough to be able to wrap all the salmon fillets.
4. Spread the ginger and spring onions on the centre of the foil tray, and place the salmon fillets on top. Pour any remaining marinade mixture over the top of the salmon.
5. Bring up the sides of the foil and fold together tightly to make a parcel so that the no moisture cannot escape during cooking.
6. Place the tray in a preheated oven at 200°C / 400°F / Gas 6 for 10–12 minutes.
7. While the salmon is cooking, place the potatoes in a large saucepan, cover with water and boil until soft.
8. Drain off all the water, and while the potatoes are still warm, add the butter and salt and mash until smooth, ensuring no lumps remain.
9. Serve the Salmon with some of the ginger and spring onions from the tray, with the warm potato mash.